1. Field of the Invention
This invention relates to a sintered body of alumina and partially stabilized zirconia (hereeinafter referred to as "PSZ"), a process for making such a sintered body and a method of preparing a powder.
2. Description of the Prior Art
A ceramic material comprising particles of alumina as a matrix and PSZ dispersed therein is expected to be of great use as a structural material because of its high toughness, bending strength, hardness, and wear resistance. This material is mainly manufactured by vapor phase reaction, coprecipitation, or the wet mixing of powders in a ball mill.
The powder produced by vapor phase reaction or coprecipitation is, however, composed of particles which are so fine and active as to agglomerate easily, though it can be sintered at a temperature which is as low as from 1300.degree. C. to 1500.degree. C. Moreover, both of vapor phase reaction and coprecipitation involve a complicated process and the product thereof is, therefore, expensive.
The wet mixing of powders is, on the other hand, a simple and inexpensive process. It, however, calls for a long time and, therefore, lacks efficiency. Moreover, the mixture requires a sintering temperature which is as high as from 1500.degree. C. to 1700.degree. C. For example, Japanese Patent Application laid open under No. 190259/1984 states that a temperature of 1500.degree. C. to 1700.degree. C. is preferable for sintering a material comprising an alumina matrix and PSZ dispersed therein, and Japanese Patent Application laid open under No. 288157/1987 describes an example in which a temperature of 1600.degree. C. was employed.
No sintered product having a high bending strength has hitherto been obtained unless its sintering is carried out at elevated pressure. For example, Japanese Patent Application laid open under No. 6126/1986 discloses a sintered product of a powder prepared by a vapor phase reaction, but it has a bending strength of only 78 kg/mm.sup.2. Japanese Patent Application laid open under No. 103859/1988 discloses a sintered product prepared from a mixture of alumina slurry and PSZ slurry but it has a bending strength of only 62 kg/mm.sup.2. The sintered composite of alumina and PSZ appearing in the Daily Industrial News (Japanese newspaper) of Dec. 6, 1989 has only a bending strength not exceeding 80 kg/mm.sup.2.
A high-pressure sintering process using, for example, a HIP (hot isostatic press) or a hot press has hitherto been employed for making a densely sintered product having fine grain size and high strength. An abnormal grain growth is likely to occur in a powder containing a large amount of impurities as a result of the formation of an amorphous phase at the grain boundary, or a liquid phase during its sintering. In either event, the abnormal grain growth results in a sintered product having low strength.
According to a first aspect of this invention, there is provided a sintered body comprising alumina and partially stabilized zirconia, and having an average grain size diameter not exceeding 0.7 micron.
According to a second aspect of this invention, there is provided a process for making a sintered body of alumina and PSZ which comprises preparing a wet mixture of a finely divided alumina powder of high purity having a primary particle diameter not exceeding 0.15 micron and an average particle diameter not exceeding 0.3 micron, and a partially stabilized zirconia powder of high purity having an average particle diameter not exceeding 0.3 micron, molding the mixed powder, and sintering the molded product at a temperature of 1300.degree.-1500.degree. C.
According to a third aspect of this invention, there is provided a method of preparing a sintering powder by the wet mixing of alumina and PSZ powders which comprises using a finely divided alumina powder of high purity having a primary particle diameter not exceeding 0.15 micron and an average particle diameter not exceeding 0.3 micron, and a PSZ powder of high purity having an average particle diameter not exceeding 0.3 micron.
These and other features and advantages of this invention will become more apparent from the following description and the accompanying drawing.